Network communications including domestic access networks are more and more based on optical fibers. When integrating optical fibers and electronics there is a need for an optical interface including a fiber coupling device of some standard design and an electronic circuit for transmitting electrical signals into optical signals and vice versa. The coupling device and the electronic circuit are connected to each other by a short optical fiber of a standard multimode or singlemode type. Normally the fiber coupling device, the short fiber connection and at least the transmitter/receiver chip of the electronic circuit are assembled to a sub-module, which in turn is to be installed in the total interface unit. The sub-module has to be treated with great care both when assembled and handled because of its sensitive connections at the two fiber ends. The fiber coupling end has to be very well polished and the chip end stripped and accurately cut and in particular the later is hard to achieve and the module is easily broken or damaged when handled in the next mounting step.